What Makes An Effective Digipak

To try and broaden my knowledge of what will make an effective digipak for Noel Gallagher's High Flying Birds, I searched the question 'what makes good album artwork?' in hope of finding people's opinions and ideas.

I found the question had already been asked on yahoo.com which was helpful, as the person who answered gave a fairly valid opinion, this will therefore aid the group and I whilst creating our own digipak as we can considor these aspects in order to make a product which attracts the target audience.